1. Field of the Invention
The present invention relates to an optical cross-connect apparatus, optical add/drop switch used in a high speed and high capacity WDM (Wavelength Division Multiplexing) system, and to a controller and method for control of an optical switch used in a wavelength router etc.
2. Description of the Related Art
A WDM scheme is promising means to establish a high capacity optical communication network and traffic in the network is dramatically increasing as use of the Internet is recently and rapidly spreading all over the world. An optical cross-connect (OXC) system in a backbone optical network is able to automatically and immediately provide transmission via a redundant fiber optic network or alternative fiber optic network upon fiber failure in order to rapidly recover the system and further has capability of determining how to distribute optical paths for individual wavelengths and converting one wavelength to another.
FIG. 5 illustrates schematically an example of an OXC system. The OXC system shown in FIG. 5 includes a plurality of optical nodes (optical signal switching apparatuses) 100 connected in a mesh and each optical node 100 is configured to include, for example, a preamplifier 101, demultiplexer (optical branching filter) 102, optical switch 103, multiplexer (optical multiplexer) 104 and post amplifier (Erbium Doped Fiber Amplifier: EDFA) 105, etc.
It should be noted that the preamplifier 101 is for amplifying a WDM signal received from an input optical transmission line to a predetermined level for all wavelengths of interest and the demultiplexer 102 is for demultiplexing the WDM signal output from the preamplifier 101 into light signals of different wavelengths (on different channels). Note that the preamplifier 101 and demultiplexer 102 are respectively provided for individual input optical transmission lines accommodated in the optical node 100.
Further, the optical switch 103 is for receiving, through a given input port, outgoing light signals of different wavelengths output from the demultiplexer 102 and outputting the signals to an arbitrary output port, thereby cross-connecting the input light signals in units of wavelengths.
Additionally, the multiplexer 104 is for wavelength-multiplexing the outgoing light signals of different wavelengths output from the optical switch 103 and outputting a WDM signal, and the post amplifier 105 is for amplifying the WDM signal from the multiplexer 104 to a predetermined level for all wavelengths of interest in order to provide transmission to the next optical node. Note that also the multiplexer 104 and post amplifier 105 are respectively provided for individual output optical transmission lines.
In accordance with such a configuration, each of the optical nodes 100 operates so that a WDM signal received from a certain input transmission line is demultiplexed into light signals (hereinafter, referred to also as channel signals) of respective wavelengths by the demultiplexer 102 and then the light signals are cross-connected in individual wavelengths by the optical switch 103. Accordingly, the channel signal input to an arbitrary input port of the optical switch 103 can be output to an arbitrary output port, i.e., an arbitrary output optical transmission line.
An example of a known OXC system includes, besides the above-stated system, an optical add/drop (Optical Add/Drop Multiplexing: OADM) ring system, as exemplarily and schematically shown in FIG. 6. The ring system is often employed in networks within a metropolitan area and a prefectural area, and has an ability to arbitrarily add and drop light signals without converting the signals of respective wavelengths to corresponding electrical signals.
It should be noted that the optical node 100 constituting a ring system shown in FIG. 6 can be configured to implement functions similar to those of said optical node 100 described in connection with FIG. 5 and in this case, the optical node is also configured to include, for example, a preamplifier 101, demultiplexer (optical branching filter) 102, optical switch 103, multiplexer (optical multiplexer) 104 and post amplifier (EDFA) 105, etc.
Moreover, in this case, it becomes possible that an outgoing light signal from a SONET (Synchronous Optical NETwork) transmission apparatus 200 or downstream router 300 (i.e., router on the side of a tributary) is added to a WDM signal transmitted over the ring system using an idle wavelength by a cross-connect operation performed in individual wavelengths in the optical switch 103, or in contrast to it, a light signal of an arbitrary wavelength is dropped from a WDM signal transmitted over the ring system and routed to a SONET transmission apparatus 200 or router 300.
Accordingly, when traffic increases on a certain site, dynamically changing assignment of wavelengths increases a bandwidth automatically to thereby increase transmissible capacity, meaning that a network configuration can be automatically altered depending on the degree of how a user is utilizing a network.
It should be noted that mainstream of the existing optical switch 103 is a switch of the type in which a light signal is first converted to an electric signal, a signal destination is switched and then again the signal is converted back to a light signal. However, when a data transmission rate is in excess of 10 Gb/s (gigabits per second) and further the number of channels increases, technique assuming the principle of opto-electric conversion cannot address the need for reduction in data transmission rate and apparatus size, and therefore, there arises a need for development of an OXC/OADM apparatus which does not depend on speed of a light signal.
Currently, an optical switch module is implemented which has a number 32 of input ports and a number 32 of output ports (32×32 channels) and an example can also be found in the Configuration of Non-Blocking Multiconnection Switching Networks (optional switch 103), in which a number of such switch modules are connected in series.
In more detail, some of optical switching devices incorporate a movable micro-mirror therein. That is, the device operates so that the orientation of the micro-mirror is controlled by an electrostatic force or electromagnetic force in order to switch the direction of a propagating light signal. Note that the above micro-mirror is formed using MEMS (Micro Electro Mechanical System) technology. The optical switch module is then constructed by two dimensionally arranging (i.e., in x- and y-directions) a number of micro-mirrors.
In contrast to such a mechanical optical switch with a micro-mirror, a non-mechanical optical switch without a movable part has also been proposed. For example, a switching device (optical deflection element) using an electro-optic effect is disclosed such as in Japanese Patent Laid-Open No. HEI9-5797 (hereinafter, referred to as Patent Document 1). FIG. 7A is a schematic plan view illustrating an optical deflection element according to this Patent Document 1 and FIG. 7B is a view of the device in a direction of an arrow A.
As shown in these FIGS. 7A and 7B, the optical deflection device disclosed in Patent Document 1 is configured so that an optical waveguide 402 with an electro-optic effect is formed on a conductive or semi-conductive single crystal substrate 401 and an upper electrode 403 is formed thereon.
Further, the upper electrode (prism electrode) 403 is formed in the shape (tapered shape) of a wedge (right triangle) having a side (hereinafter, referred to as a bottom) 403a orthogonal to an optical axis of incoming light and a side (hereinafter, referred to as an oblique line) 403b obliquely intersecting the optical axis.
In the optical deflection element constructed as described above, light enters the optical waveguide 402 from the side of the bottom 403a of the upper electrode 403 and exits the oblique line 403b of the upper electrode 403. Then, when a voltage is applied between the substrate 401 as a lower electrode and the upper electrode 403, the refractive index of a region of the optical waveguide 402 below the upper electrode 403 is changed, causing the refractive index of that region to become different from that of the surrounding region. This, in turn, causes light propagating through the waveguide 402 to be refracted by the region corresponding to the change in the refractive index, thereby changing the direction of light propagation. That is, changing a voltage applied between the upper electrode 403 and the substrate 401 allows control of the direction of outgoing light.
Moreover, as described, for example, in Japanese Patent Laid-Open No. 2002-318398 (hereinafter, referred to as Patent Document 2), a proposal has also been made to dispose the above upper electrode 403 on incoming and outgoing sides so that these electrodes face each other, in order to downsize an optical switch that uses an electric electro-optic effect.
FIG. 8 is a schematic plan view of an optical switch module (hereinafter, referred to also as a known example 2) proposed in such Patent Document 2 and the optical switch module shown in FIG. 8 is configured to include an optical waveguide section 501 on the incoming side, collimator section 502, optical deflection element section 503 on the incoming side, common optical waveguide 504, optical deflection element section 505 on the outgoing side, light condensing section 506 and optical waveguide section 507 on the outgoing side. Note that the optical waveguide section 501 on the incoming side, collimator section 502, optical deflection element section 503 on the incoming side, common optical waveguide section 504, optical deflection element section 505 on the outgoing side, light condensing section 506 and optical waveguide section 507 on the outgoing side are integrally formed on a substrate.
In this case, the optical waveguide section 501 on the incoming side is configured to include a plurality of optical waveguides (cores) 501a serving as an input port and a cladding layer 501b that surrounds these optical waveguides 501a and provides confinement of light within the optical waveguide 501a by using a difference in refractive index. Likewise, the optical waveguide section 507 on the outgoing side is also configured to include a plurality of optical waveguides (cores) 507a serving as an output port and a cladding layer 507b that surrounds these optical waveguides 507a and provides confinement of propagating light within the optical waveguide 507a by using a difference in refractive index.
It should be noted that the number of the optical waveguides (input ports) 501a of the optical waveguide section 501 on the incoming side and the number of the optical waveguides (output ports) 507a of the optical waveguide section 507 on the outgoing side are the same (n). That is, in this case, the optical switch module is an n×n array of optical switches. Note that needless to say, the number of the optical waveguides 501a and the number of the optical waveguides 507a may be different.
The collimator section 502 is for individually collimating light within each of a plurality of light signals incoming from the respective optical waveguides 501a of the optical waveguide section 501 on the incoming side and therefore is configured to include a number n of collimator lenses 502a. Individual collimator lenses 502a are disposed at a position apart slightly from the edges of the optical waveguides 501a, respectively. This allows light emitted from the optical waveguide 501a to be collimated by the collimator lens 502a even though the light emitted therefrom spreads in a radial pattern.
The optical deflection element section 503 on the incoming side is for individually switching the direction of each of propagating light signals, which have passed through the collimator section 502, using an electro-optic effect (Pockels effect) and a number n of optical deflection elements 503a each are disposed at a position apart slightly from the collimator lenses 502a along an optical axis. The individual optical deflection elements 503a each are comprised of a single prism pair or a plurality of prism pairs and the prism pair is formed by a method including: providing an optical waveguide (slab waveguide) 402 formed of a material exhibiting an electro-optic effect, such as PLZT ((Pb, La)(Zr, Ti)O3); forming an electrode in the shape of a wedge (e.g. triangle shape); disposing the electrodes as the aforementioned first and second upper electrodes 403 (403a, 403b) on a light signal region of the slab waveguide 402, so that the distal ends of the wedges are pointing opposite directions; and disposing the electrodes as the aforementioned first and second lower electrodes 401 (401a, 401b) below the corresponding upper electrodes.
The common optical waveguide section 504 allows light passing through the optical deflection element section 503 on the incoming side to propagate into the optical deflection element section 505 on the outgoing side. Although a plurality of light signals simultaneously pass through the common optical waveguide section 504, these light signals travel straight in previously established directions within the common optical waveguide section 504 and therefore travel without interfering with other light signals. Further, an example of an optical path between the optical deflection element section 503a on the incoming side and the optical deflection element section 505a on the outgoing side is schematically shown in FIG. 9.
The optical deflection element section 505 on the outgoing side is for individually switching the direction of each of the propagating light signals after their passage through the common optical waveguide section 504, using an electro-optic effect and similarly to the optical deflection element section 503 on the incoming side, a number n of optical deflection elements 505a are provided. These optical deflection elements 505a each have the same or similar configuration as the optical deflection elements 503a and deflect light, which has reached the optical deflection element 505a through the common optical waveguide section 504, in the direction parallel to the optical waveguide 507a. 
The light condensing section 506 is comprised of a number n of light condensing lenses 506a and these light condensing lenses 506a condense light that has passed through the optical deflection elements 505a, causing light to be guided into the optical waveguides 507a. 
According to such optical switch module, the optical deflection element sections 503 and 505 are each operable to change the propagation direction of light between the first upper electrode 403a and the first lower electrode 401a, and further to change the propagation direction of light between the second upper electrode 403b and the second lower electrode 401b, and therefore, there will be the benefit of being able to significantly change the propagation direction of light.
Additionally, since the first and second upper electrodes 403a, 403b are disposed so that the distal ends of the corresponding wedges are pointing opposite directions and the first upper electrode 403a is disposed to face the first lower electrode 401a, and the second upper electrode 403b is disposed to face the second lower electrode 401b, it can also be concluded that this approach has the benefit of providing greater geometric density of electrodes. Note that other behavior and effects of this optical switch are described in detail in Patent Document 2 and therefore explanation thereof is omitted.
However, in the optical switch module having the configuration described in this Patent Document 2, a variety of variation factors such as temperature dependence and drift over time of an electro-optic constant, and temperature dependence of optical coupling system, etc., sometimes prevent the module from providing sufficient optical coupling efficiency, even when an optimal voltage (to be applied between the electrodes 401 and 403) for providing maximum optical coupling efficiency has been pre-configured as an initial setting according to analysis of optical switch module after its fabrication.
For example, in the optical switch module described in the Patent Document 2, when the collimator section 502 on the incoming side is displaced by 50 μm in the direction orthogonal to the direction of incoming light, optical coupling efficiency is reduced by 5 dB. Further, when atmospheric temperature changes, it is expected that a difference between thermal expansion coefficients of the collimator section 502 and the common optical waveguide section 504 causes displacement of the above optical system. Moreover, in the optical deflection elements 503a, 505a formed of a material exhibiting an electro-optic effect as is the case with the optical switch module of this Patent Document 2, deflection angle versus applied voltage characteristics potentially changes with the lapse of time or due to temperature changes.
The foregoing description indicates that the optical switch module disclosed in Patent Document 2 needs a method for monitoring optical output power and using feedback control so that variations in the optical output power are prevented. Subsequently, for explanation of the conventional examples, an angle at which an optical beam is deflected by the wedge-shaped electrode will be discussed. When wedge-shaped electrodes are disposed facing each other on the upper and lower side of the slab waveguide that exhibits an electro-optic effect and has a thickness of d, and a voltage V applied between the upper and lower electrodes, a refractive index change Δn due to the first order electro-optic effect (Pockels effect) is given by the following equation (1):
                              Δ          ⁢                                          ⁢          n                =                              -                          1              2                                ⁢                      r            ·                          n              3                        ·                          V              d                                                          (        1        )            where r is an electro-optic constant (Pockels constant: TE mode) in the direction of electric field, n is a refractive index for abnormal light. Further, as shown in FIG. 10A, when assuming that an incident angle to the wedge-shaped electrode is θin, a deflection angle of light at the input plane is α, an outgoing angle of light is θout, and paraxial ray approximation is valid for all of θin, α and θout, the relationship given by the following equation (2) is established between θin and θout:
                              θ          out                ≅                              θ                          i              ⁢                                                          ⁢              n                                -                                    L              W                        ·                                          Δ                ⁢                                                                  ⁢                n                            n                                                          (        2        )            
Moreover, in the case of a prism pair (see FIG. 10B), the corresponding relationship is given by the following equation (3):
                              θ          out                ≅                              θ                          i              ⁢                                                          ⁢              n                                -                      2            ·                          L              W                        ·                                          Δ                ⁢                                                                  ⁢                n                            n                                                          (        3        )            
Subsequently, a Gaussian beam model is applied to the optical system shown in FIG. 8 and optical coupling efficiency between input and output fiber optics is computed. As shown in FIG. 11, it is assumed that an optical reference plane 700 is the midpoint between input and output; lateral direction, longitudinal direction, and direction vertical to the paper are z-, x- and y-axes, respectively; and a deflection angle is so small that paraxial ray approximation is valid. Moreover, input and output are symmetric relative to the reference plane 700 and the spot sizes of incoming light and outgoing light are the same, and distances from the reference plane 700 to beam waists are equal. In this case, the optical coupling efficiency η of a Gaussian beam is represented by the following equations (4) and (5):
                    η        =                              k            x                    ⁢                      exp            ⁡                          [                                                -                                      k                    x                    2                                                  ⁢                                  {                                                                                    Δ                        ⁢                                                                                                  ⁢                                                  x                          2                                                                                            w                        2                                                              +                                                                                                                        π                            2                                                    ⁢                                                      Δθ                            2                                                    ⁢                                                      w                            2                                                                                                    λ                          2                                                                    ⁢                                              (                                                  1                          +                                                                                    (                                                                                                λ                                  ⁢                                                                                                                                          ⁢                                  z                                                                                                  π                                  ⁢                                                                                                                                          ⁢                                                                      w                                    2                                                                                                                              )                                                        2                                                                          )                                                              -                                          ΔχΔθ                      ⁢                                                                        2                          ⁢                          z                                                                          w                          2                                                                                                      }                                            ]                                                          (        4        )                                          k          x                =                              {                          1              +                                                (                                                            λ                      ⁢                                                                                          ⁢                      z                                                              π                      ⁢                                                                                          ⁢                                              w                        2                                                                              )                                2                                      }                                -                          1              2                                                          (        5        )            
In the above equations, λ is a wavelength, w is a beam waist width, z is a distance from the reference plane 700 to the beam waist. Further, Δx is a displacement between propagating light and a nominal optical axis in the reference plane 700, and likewise, Δθ is an angular displacement of the propagating light from the nominal optical axis.
Given that a voltage applied to the prism pair 503a on the incoming side is Vin and a voltage applied to the prism pair 506a on the outgoing side is Vout, it is ideal that when an optical path from a certain input channel m to a certain output channel n is established, the relationship Vin=Vout results. That is, a beam exiting the collimator section 502 and deflected by the prism pair 503a on the incoming side is deflected by the prism pair 505a on the outgoing side at an angle parallel to the beam exiting the collimator section 502, and enters the light condensing section 506. Here, it is assumed that the values of Vin and Vout are an initial value (path information) upon establishment of the path. The path information can be given by referring to a memory.
As described above, although it is expected that variation factors such as temperature changes and drift over time act to displace the above initial values from values corresponding to the optimal optical coupling, the module comes into a state in which optical power can be sufficiently detected (monitored). Under such state, it becomes possible that feedback control is applied to Vin, Vout. Control allowing optical coupling efficiency to be maximum (i.e., optimal coupling control) is performed so that Vin or Vout is finely adjusted so as to have a value which causes the optical power detected to be maximized using the feedback control.
It should be noted that when optical coupling efficiency versus applied voltages Vin, Vout is calculated, a distribution such as that shown in FIG. 12A can be obtained. FIG. 12B is a diagram illustrating a contour map for the distribution of optical coupling efficiency shown in FIG. 12A. Note that in FIGS. 12A and 12B, the optical coupling efficiency is normalized and the contour map shows an inclined elliptical distribution in (Vin, Vout) coordinate system.
In a control system having such a distribution of optical coupling efficiency, a conventional control scheme has been implemented so that feedback control is performed by alternately and finely adjusting Vin and Vout. That is, the conventional control scheme is to control the contour map along Vin axis and Vout axis. According to this scheme, an identification failure zone will be produced so as to correspond to a region in which search is performed in a direction to a peak point. An example of the identification failure zone is shown in FIG. 13.
First, a process of how a point of interest reaches a peak point (point P) from a point X in FIG. 13 will be analyzed below. At the beginning, feedback control is selected along a cross section taken along a line A–A′ parallel to Vin axis and optical output power (optical coupling efficiency) after application of a unit step of ΔVin is compared to the power just before the application of the unit step, and Vin is changed so that the power increases. When the power exceeds the peak X′ after application of a few steps of ΔVin, the control performed parallel to Vin axis is temporarily terminated and transferred to a control performed parallel to Vout axis.
Likewise, the control performed parallel to Vout axis is performed in a unit step of ΔVout and terminated when the power exceeds the peak Y in the plane of a cross section taken along a line B–B′. Such operation is repeated to find the peak point P. The peak point P is determined so that the power necessarily decreases whenever the point of interest moves toward a positive (+) or negative (−) direction of Vin and Vout axes, and after determination of the peak point P, the search is completed.
The aforementioned feedback control algorithm will be explained with reference to FIG. 14. First, input/output channel information is retrieved from a memory (step A1); prism pairs 503a, 505a to be controlled are selected and applied voltages Vin, Vout (initial values) to the selected prism pairs 503a, 505a are determined (step A2); and the determined voltages are applied to the prism pairs 503a, 505a (step A3).
Then, the optical output power is monitored and a received power level is detected (detection of outputs from an A/D converter, etc.) (step A4); and whether or not an abnormality including, for example, the fact that the optical output power cannot be detected occurs is determined (step A5). When the abnormality has occurred (in case of NO at step A5), the process beginning with the above step A1 is again implemented. That is, it can be concluded that the aforementioned process is feedforward control which is performed to determine, based on the input/output channel information, initial voltages to be applied to the prism pairs 503a, 505a to be controlled.
On the other hand, when no abnormality has been detected in the monitored optical output power (in case of YES at step A5), the process is transferred to subsequent feedback control. That is, first, an applied voltage Vin to the prism pair 503a on the incoming side is increased by a voltage of ΔVin (step A6) and a received power level (A/D value) is detected (step A7). When the A/D value increases (in case of YES at step A8), the process determines that the current search direction is correct and Vin is increased by a voltage of ΔVin (step A9). In contrast to it, when the A/D value decreases (in case of NO at step A8), the process determines that the current search direction is incorrect and Vin is decreased by a voltage of ΔVin (step A12).
After that, a number of repetitions of Vin increase is performed (YES route at step A10 or step A14, and at step A11 or A14) until the A/D value begins to decrease (until NO is determined in step A11 or step A14) and when the A/D value has decreased, the control along Vin axis is stopped and the process is transferred to control along Vout axis (control of an applied voltage to the prism pair 505a on the outgoing side) (step A15).
The control along Vout axis is also implemented in a manner similar to the control along Vin axis (step A16 to step A24) and again, the process is transferred to the control along Vin axis (step A25). After a predetermined number (N) of repetitions of the above loop (NO route at step A26), the search to find the peak point is completed (YES route at step A26).
However, although a shortest course to be detected by the control is a path denoted by the numeral 602 in FIG. 13 according to the aforementioned control scheme, in the case of a start point X being located within the identification failure zone 600, the direction of starting the search is just opposite to that of starting the search to find the shortest course 602 (refer to a path 601). Consequently, the search will be performed taking a lengthy detour to reach the peak point P until the search is converged and therefore it takes a long time to implement the feedback control, significantly delaying the switching of optical paths in the optical switch module.